Glenn Greenwald, the Guardian journalist who first published secrets leaked by former NSA contractor Edward Snowden, promised Monday to release more documents, saying the UK would be “sorry” for detaining his partner for nine hours.

The journalist’s partner, David Miranda, was held by British
authorities under anti-terrorism laws while transiting through
London’s Heathrow Airport on his way to Rio de Janeiro on Sunday.

The US government has released a statement saying that British
officials told them about their decision to detain Miranda,
although Washington denied its own involvement.

"This is a decision they made on their own," White House
spokesman Josh Earnest told reporters at a briefing.

The move by UK authorities sparked a furious response from
Greenwald.

"I have many more documents to report on, including ones about
the UK, where I'll now focus more. I will be more aggressive, not
less, in reporting," said Greenwald, speaking in
Portuguese to reporters at Rio de Janeiro's international
airport, Reuters reported.

"When they do things like this, they show the world their real
character. It'll backfire. I think they'll come to regret
it," he said.

Miranda described being detained like a terrorist by UK
officials.

“They treated me like I was a criminal or someone about to
attack the UK…they were threatening me all the time and saying I
would be put in jail if I didn't cooperate,” Miranda told the
Guardian. “It was exhausting and frustrating, but I knew I
wasn't doing anything wrong."

There was also angry reaction from Brazilian authorities, as well
as from journalists and human rights activists in the UK.

The Brazilian government said in statement that Miranda’s
detention was “without justification.”

The Guardian said it was dismayed at the detention of Miranda and
was “urgently seeking clarification from the British
authorities” as to why it happened.

On Tuesday Greater London’s Metropolitan Police Service (MPS)
said the detention of Miranda was both “legally and
procedurally sound.”

Saying the detention of the 28-year-old was “necessary and
appropriate”, the MET continued that contrary some reports,
Miranda had been offered legal representation and was attended to
by a lawyer.

"No complaint has been received by the MPS at this time,"
the police statement read.

The statement continued that Schedule 7 of the Terrorism Act
2000, which grants police the power to stop and question people
traveling through British ports and airports to determine whether
they are involved in planning terrorist plots, was used
"appropriately and proportionately".

Earlier, Keith Vaz, a Labor lawmaker who chairs parliament's
powerful interior affairs committee, told the BBC that he had
written to the head of London's Metropolitan Police to ask for
clarification of what he labeled an "extraordinary" case.

“Now you have a complaint from Mr. Greenwald and the Brazilian
government. They have said that they are concerned at the use of
terrorism legislation for something that does not appear to
relate to terrorism. So it needs to be clarified and clarified
quickly,” said Vaz.

Shadow Home Secretary Yvette Cooper said that “swift
answers” were needed as to why Miranda was detained for so
long.

“Any suggestion that terror powers are being misused must be
investigated and clarified urgently - the public support for
these powers must not be endangered by a perception of
misuse,” she said.

Labour MP Tom Watson called for parliament to look into what he
said could be an attempt to “get the genie back into the
bottle” when it returns from summer recess in September.

He also questioned whether government ministers had been briefed
of the move which he said was “clearly an embarrassment for
the government.”

Michael Mansfield, one of Britain's leading human rights lawyers,
told Reuters that the action amounted to an act of oppression.

“The detention of David Miranda is a disgrace and reinforces
the undoubted complicity of the UK in US indiscriminate
surveillance of law-abiding citizens. The fact that Snowden, and
now anyone remotely associated with him, are being harassed as
potential spies and terrorists is sheer unadulterated state
oppression," he said.

Widney Brown, Amnesty International senior director of
international law and policy, questioned what danger Miranda
could possibly be to the UK government.

“He was detained under a law that violates any principle of
fairness and his detention shows how the law can be used for
petty vindictive reasons. There is simple no basis for believing
that David Miranda presents any threat whatsoever to the UK
government,” she told the Daily Mail.

Journalists groups have also accused the authorities of misusing
their powers against "terrorism."

“Journalism may be embarrassing and annoying for governments
but it is not terrorism,” Bob Stachwell from the Society of
Editors told the BBC.

‘A total abuse of power’

Miranda, who is a Brazilian citizen, said he was questioned for
nine hours by numerous agents before being released without
charge. Authorities seized his laptop, mobile phones, watches,
and flash drives.

"They got me to tell them the passwords for my computer and
mobile phone," Miranda said. "They said I was obliged to
answer all their questions and used the words 'prison' and
'station' all the time."

He said that seven British agents questioned him on all aspects
of his life and only released him after he started shouting in
the airport lounge. "They even asked me about the protests in
Brazil, why people were unhappy and who I knew in the
government," said Miranda.

The 28-year-old was traveling from Berlin to Rio de Janeiro after
visiting US filmmaker Lauro Poitras, who has been working on the
Snowden files with Greenwald and the Guardian.

Miranda described being detained as soon as he stepped off the
airplane Sunday morning. "There was an announcement on the
plane that everyone had to show their passports. The minute I
stepped out of the plane they took me away to a small room with
four chairs and a machine for taking fingerprints," he
recalled.

He said that he was not allowed to call his partner, Greenwald,
who is a licensed lawyer in the US. Miranda added that he was not
allowed an interpreter, despite having trouble speaking in his
second language.

"I was in a different country with different laws, in a room
with seven agents coming and going who kept asking me questions.
I thought anything could happen. I thought I might be detained
for a very long time," he said.

The Daily Mail reported that Miranda was carrying USB drives of
encrypted documents from Edward Snowden.

Greenwald didn’t confirm what Miranda was carrying but said that
only he and Poitras have “copies of the full archives of NSA
documents which Edward Snowden gave to journalists, so much of
the speculation about what [Miranda] was or wasn’t carrying is
misinformed.”

Miranda said it was clear why he was detained. "It's because
I'm Glenn's partner. Because I went to Berlin. Because Laura
lives there. So they think I have a big connection," he said.
"But I don't have a role. I don't look at documents. I don't
even know if it was documents that I was carrying. It could have
been for the movie that Laura is working on."

The Metropolitan Police confirmed that a 28-year-old man was held
from 08:05 BST until 17:00 BST, under Schedule 7.

"This law shouldn't be given to police officers. They use it
to get access to documents or people that they cannot get the
legal way through courts or judges. It's a total abuse of
power," added Miranda. "I have friends in the UK and liked
to visit, but you can't go to a country where they have laws that
allow the abuse of liberty for nothing.”

British authorities confirmed Miranda’s detention under an
anti-terrorism law, but did not elaborate on the reasons for
their actions. However, the UK Home Office has defended Schedule
7 in the past.

David Anderson, the official independent reviewer on the UK’s
terrorism legislation, said he asked authorities why Miranda was
detained for so long. Of the 69,000 people detained under
Schedule 7 in 2011-2012, only 40 were held for six hours or more.
In most cases, people are released after less than an hour.

Snowden, who has been granted asylum by Russia, gave Greenwald up
to 20,000 documents with details about the US National Security
Agency and the UK’s GCHQ surveillance operations.